How horrible bad is it to take a numbers matching 68. 302 and turn it into a 343 stroker engine
Butchery.
But as many others will say. It’s your car do what you want with it! 
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Get a roller 5.0 from a junkyard explorer and save the original 302.
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There’s the logical side of the argument. If the car is a rarity, preserve it. If the car is the standard garden variety Cougar, do what you want and don’t feel bad about it. That’s how I treated my garden variety car…
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I think it’s a great idea. It can be invisible if you want.
We took the 289 out of my father’s '66 Mustang and replaced it with a 302 stroked to 347. We put all of the 289 front bits on so you can’t tell that it isn’t a 289. After about 15 years of storage he got rid of the original 289 black assembly.
I say go for it, then again I’m biased as I want to do the same thing but my issue is my motor runs fine and I don’t want to down it for a motor build…yet
It is seized up and ready to go through top to bottom so I’m thinking why not
It’s sort of the route I went. If you’re willing to spend the money to machine and rebuild instead of replace, it’s worthwhile.
Just a few tips:
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Do not use a flat tappet cam. Yes, they used to be fine. If you have one, they can still work fine, even with modern oils. But if you buy new flat tappet lifters, you are going to get to rebuild your engine twice. New lifters are not okay. You have been warned. Instead, go with a roller cam, linkbar lifters, and appropriate springs.
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Heads are the biggest bottleneck for any Windsor. More cubic inches with stock heads will deliver more torque, more grunt as you get rolling, but will also poop out faster as they run out of breath. If you don’t change your heads, you will likely have about the same level of overall performance as it did before.
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Pistons make a big difference. Run flattops if you can. (valve reliefs are fine) If you run a dished piston to reduce compression, it will also ruin your quench, leading to increased octane sensitivity and worse performance. Quench is critical. You should try to get your piston-to-head clearance in the neighborhood of .037-.045 if possible. If you run lower rpms and have good parts, limiting piston rock, rod stretch, etc. you could get away with possibly as little as .035, but that gets a little sketchy. If you need to, you can use reverse dome pistons, which mimic the combustion chamber shape and keep your quench area intact, unlike the generic bowl of a dished piston. Mahle has superior design, low weight, and very low friction thin rings that work great. Icon and Autotech/Speedtech are also great buys.
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Carburetion or EFI? If you intend to run a carburetor, and want this for a street machine, you’d be hard pressed to do better than a good dual plane intake (Weiand Stealth, Edelbrock Performer RPM) and a 600CFM Summit M2008VS carb. Yes, annular boosters are a BIG deal. For EFI, look to Fitech or Holley Sniper for some good solutions.
Depending on your intentions, you might discover that a good set of heads and some steeper rear gears along with your factory crank will actually work very well too. The stock Ford 289 and original 302 cranks are nice and light, which allows the engine to accelerate faster. The tradeoff is that they will produce less torque. If you pair a lightweight rotating assembly with a heavy car that has 2.79 gears out back, it will not offer much of a benefit. Our cars are fairly light (especially when compared to more modern cars), so when combined with 3.55 or higher rear gears, a little 289 can really rip, if you give it better heads, cam, headers, and intake.
The longer crank throws of a stroker work in much the same way as higher numeric rear gears. More leverage on the crank means more torque delivered to the rear tires, at the expense of running out of power sooner at higher RPMs/speeds.
My car is going to be on the highway and probably dailied, so I chose a 331 to increase torque for my automatic, but keep piston speed reasonable. I went with a Scat 9000 crank, Mahle PowerPak reverse dome pistons to keep my compression down (9.3:1). Economy and low-midrange performance were my goals, with the intent of being all done by 5500 or so. To that end, the cam I picked is a 1985 5.0 HO stock grind (the year is not really anything special - the Mustang 5.0 cams are all mostly the same). To help feed the extra cubic inches, I went with Flowtech heads with 58cc chambers and small intake ports, along with 1.7 roller rockers. I should have smooth idle, excellent chamber pressure right from idle to redline, and excellent economy.
It’s being paired with a 4R70W transmission, and 3.55 rear gears. The old 5.0 HO was almost stock and put down 230 horsepower to the rear tires, getting 21-23 mpg with the C4 and 2.79 rear gears. I’m expecting something north of 350 horsepower with this build, and hopefully 25+ mpg out on the highway.
Hope this helps!
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Man that sounds nice I’d love to have anything above or around 340 HP ; but of course I have two of them to do and not really ready till after the first of the year ; so Cost might come into play as to which way I go
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over the years i have thrown a mountain of cash at go fast motors to eventually sell them for no were near what i put into them.
My advice is : if your existing OE motors is still running ok, then absolutely pull it out and thro a blanky over it and leave it alone.
Wen time comes to sell the car it will be worth more and more saleable with the OE motor, and you could cash up the go fast motor or transplant into another project.
Buyers get all warm and fuzzy when you mention the phrase “ numbers matching”
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Sound advice!
With my car, there’s no chance I’ll sell it. I’m chasing a dream that I’ve held for most of my life, and the project will never end. I just keep making it the car I really want. Maybe that makes it desirable to others too, but I’ve come to the conclusion that it’s for me and no one else.
Well, maybe my wife, at least a little. I do love going places WITH her, and she’s hated most of my other vehicles. =) She’s amazing, and I love her deeply, despite her not being a “car person”.
If you’re not as crazy nor passionate about your car as I am, and might actually sell it at some point, then socking away the original motor and throwing in something else that matches your goals makes great sense!
never say never ………lol
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This car has been through a divorce, moved from Kansas to California, had an engine fire, sat in an open shed next to a field for several years, moved back to Kansas non-running, moved to Utah non-running, went through another divorce, stayed in my friend’s garage in Utah for 3 years, moved back to Kansas non-running, and has been here for another 8 or so years. Now I’m getting her running again, finally.
While it’s possible I won’t have this car forever, I’ll be darned if I willingly give it up. We’ve been through a lot together, even if most of it was with her sitting there with her motor all burned up. Both of us could use some work, but there’s plenty of miles left in us! 
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The stories these cars could tell. And they will still be here when we are gone
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You can always go back to a stock spec rotating assembly if you absolutely had to return it to an original 302. Myself I purchased a roller 302 block I’m planning to have built as a 347, the original longblock will be kept in the garage.
I would rebuild the original engine. The 302 was a good engine, the 289 was even better. I would not go the 343 unless you are going drag racing. The fondness, economy and reliability of the 289/302 will outlast the the quick thrill of the torque of the 343. If anything I would go 302 with long rod combination, ported cast heads. I prefer the 289 (out to 290) with longer rods, can easily make 500 + Hp on the dyno with that combination. Same thing with increased fuel economy. 
Peter
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Quench is critical. You should try to get your piston-to-head clearance in the neighborhood of .037-.045 if possible. If you run lower rpms and have good parts, limiting piston rock, rod stretch, etc. you could get away with possibly as little as .035, but that gets a little sketchy.
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Experience tells me I would not do this. I was running similar clearances between heads and pistons on my 302 windsor, (I think I started at 0.39). I had a lot of trouble with blowing head gaskets. I increased the head bolt torque, then the head gasket thickness to help, and it stopped the head gaskets blowing, but pistons were still hitting the head. I then started bearing blueing the pistons to see where they hit the heads, then removed material from pistons to increase clearance. I finally got it to work, but it did not solve the ring seal problem at the top of the cylinder. That is, when the pistons hit the head, it caused the pistons to rock violently causing bad ring wear in the top of the cylinder bore. In my opinion, having good ring seal is better than tight piston to head clearances. 
Strokers are GREAT…have a few…race a few…unless you are planning on “Ok” chalk marks and correct color " Caution Fan" decals the interior of the engine is something, only your right foot abd next ex wife will notice. I drag race stroker Clevelands and have mad love for my 393.
I love all this talk about quench and piston speed. You want glass smooth torque and too much power …and… stay married…brutha…just drop a 351W in there with Cougar finned valve covers, a good ball collector header, RPM Air Gap intake with a good 650 cfm vacuum secondary carb…good ignition. I bet almost NO ONE will be questioning the “302 High Performance” decal on the air filter. I love taking people fir a ride in my 69 vert and demonstrating the proper operation of the kickdown lever. Then asking them if they think it has a 750 cfm or 600 cfm carb…no one EVER guesses …that there is a stock 2V carb H code Windsor under the hood. Properly tuned 351W will be easy to complete,easy to get parts, and ez on the wallet…and will allow you to drive away from and not too…the court house
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This is not an either-or thing. Good ring seal is important, head gasket seal is important, and quench is important.
You can’t have any of these without considering all of the other factors, like compression ratio and cam choice. Running a very high dynamic compression ratio will always cause problems on pump gas.
However, tight quench helps in a lot of ways. It effectively adds octane tolerance to your engine, allowing you to get better performance, run cooler, and use cheaper gas. .037-.045 is a comfortable range, and should not cause problems with a typical Windsor. If you were hitting the heads at .039, that indicates that perhaps you had a lot of piston rock, were running BIG rpms, did not have the block/heads straight and true to each other, or perhaps the head gaskets were not the thickness you thought they were.
This is not new tech stuff. Sir Harry Ricardo was describing the effects of quench, and laid out excellent guidelines for piston-to-head clearance clear back in the 20s.